The purpose of this project is to delineate the cellular mechanisms which explain the selective differentiation of IgA B cells in mucosal lymphoid follicles, Peyer's patches. Initially, we showed that B cells bearing surface IgA (sIgA B cells) arise from B cells bearing surface IgM (sIgM B cells) only when the latter are co-cultured with a particular kind of T cell derived from the Peyer's patch, so-called switch T cells. The latter were found in Con A-stimulated clonal T cell populations and required a polyclonal stimulant (LPS) to exert their effect. The switch T cells did not act on surface IgG-bearing T cells and did not cause sIgA B cells to undergo terminal differentiation into IgA-producing plasma cells. Thus the switch T cells were class-specific and appear to act at a fundamental level to influence the coursse of Ig-gene rearrangements. One of the important unanswered questions concerning the switch T cell is whether it is an antigen-reactive cell that brings about switch at the same time it mediates more conventional helper T cell functions, or whether it is an auxiliary cell which is activated during the course of an antigen-driven immune response, but is not itself stimulated by antigen. In the present series of experiments we addressed this question by studying the capacity of switch T cells to proliferate in culture as a result of exposure to autologous and allogeneic cells of various sources. We found that cloned T cells obtained from Peyer's patches and having switching capacity, proliferated in culture when exposed to Con-A activated T cells, LPS-activated B cells and LPS-activated macrophages, but not to unstimulated T cells. This stimulation was blocked by addition to the culture of monoclonal antibodies against appropriate I-A antigens but not antibodies against inappropriate I-A antigens or antibodies against D/K antigens. Thus, the switch T cells do in fact proliferate when exposed to autologous I-A antigens and are thus autoreactive cells. These results make it very likely that switch T cells are auxiliary cells that are activated by autologous I-A antigens that are exposed during cell activation induced by antigen.